Traditionally, natural casings of animal intestines, and also synthetic casings on the base of collagen and cellulose, having high steam and smoke permeability, are used as casings for smoked sausage products. However, natural intestine casings are an expensive and scarce product, and furthermore are subject to bacterial spoilage. The production of cellulose and collagen synthetic casings is based on low-productive and multistage solution technologies related to the use of such toxic materials as carbon disulfide and formaldehyde and presumes a large amount of harmful liquid wastes. This results in a relatively high cost of such casings.
Due to the above-mentioned reason, a desirable aim is the development of synthetic analogues of the described casings, which analogues would be produced by highly productive extrusion methods.
Linear aliphatic polyamides or mixtures thereof with other polymers such as an ionomer resin, modified ethylene vinyl acetate copolymers and/or modified polyolefins are disclosed in EP No. 0 139 888, published 8 May 1985 as polymer materials suitable for making smokable casings. Wherein, the glass transition temperature of the polymer material decreases depending on the absorption of water. Under conditions of “wet” smoking the products obtain proper fragrance and taste. These casings readily pass the smoke substances, have excellent mechanical properties and low gas permeability. However, due to low vapor permeability they cannot be used instead of natural, collagen or cellulose casings and they purport obtaining therein a somewhat different, more moist product than traditionally smoked sausage.
A casing for smoked food products, suitable for both smoking and for storing therein cured products, i.e. having high barrier properties in respect to oxygen and vapor, is disclosed in U.S. Pat. No. 5,084,310, published 25 Jan. 1992. The packaging material comprises at least one layer of a mixture of 5-60 wt. % of polyvinylidene chloride (PVDC) and 95-40 wt. % of polyamide. In order to increase the gas barrier properties of the film, PVDC is introduced into a smokable polyamide. A copolymer, mainly consisting of vinylidene chloride, preferably a copolymer of 65-98 wt. % of vinylidene chloride and 2-35 wt. % of a monomer capable of forming a copolymer with vinylidene chloride, for example, vinyl chloride, (met)acrylic acid, acrylonitrile, is used as the PVDC. A polyamide, having a low melting point, not exceeding 210° C., is used as the polyamide to be mixed with the PVDC. Nevertheless, this casing does not pass the smoke substances to a sufficient degree and has high barrier properties in respect to water vapors. A sensor test on the capability for the products to be smoked in this casing shows fragrance and taste of the smoked product to be approximately at the level of a casing of pure polyamide but significantly worse than in the case of a cellulose casing.
In U.S. Pat. No. 4,851,245, published 25 Jul. 1989, smokable films are produced from a mixture of 50-80% polyamide (PA) with a melting point of 120-210° C. and 20-50% of a copolymer of ethylene and vinyl alcohol. Polyamide 6.66, polyamide 612 or mixtures thereof are claimed as the polyamides. The packaging film has good smoke permeability, which in this patent is evaluated according to the permeability in respect to vapors of methanol, but it was not the object of this invention to increase the moisture permeability of this film, made in the form of a tubular casing. Moreover, in one of the embodiments of the invention the casing comprises a moisture impermeable polyolefin layer.
A film for smoking and storage is disclosed in EP No. 0 252 597, published 13 Jan. 1988. This film is made of a blend of polymers in an amount of 80-98 wt. % and additives in an amount of 2-20 wt. %, wherein the polymer mixture consists in turn of 30-100 wt. % of polyamide and 0-70 wt. % of polyolefins. This additive that actually is a plasticizer, is compatible with the polymer components, dissolves in water and/or oil and is in a liquid state at 70-95° C., which makes it possible for it in the process of successive heating to sweat in the inner and/or outer surface of the film, after which the film becomes vapor and gas impermeable, as if it is made from the same polymer materials, but with no additive. The following are used as an additive: aliphatic alcohols, polyglycols, esters of polyoxyethylenesorbitane and fatty acid, esters of polyatomic alcohols, esters of dibasic aliphatic acids, esters of polyvalent oxycarboxylic acids, ethers of aliphatic acids and epoxy plasticizers. Impairment of the mechanical properties, which may result in rupture of the film when sausage is being stuffed into the casing, relates to drawbacks of these strongly plasticized films. Furthermore, premature sweating of the plasticizer on the outer surface may result in impairment of the printing ink adhesion, and inside—may promote the acquirement of a foreign taste by the product packed in the film.
The most pertinent prior art is EP No. 0 920 808, published 9 Jun. 1999 which discloses a film used for packing cooked processed meat products, such as sausage and ham, and also for cheeses, in the case where they are smoked and subjected to heat treatment. The film is prepared either from acetate-propionate cellulose or a mixture thereof with polyamide with a weight ratio of the first to the second being 5-90:95-5, preferably 5-30:95-70 with a thickness of from 5 to 90 μm, wherewith the film in accordance with the invention has a permeability in respect to water vapor equal to 300-363 g/m2 per day. However, the films described in the examples, which contain polyamide as the main component, have the permeability, which is not more than 300 g/m2 per day. The polyamide may be PA 6.66, PA 11, PA 12, PA 612 or a mixture thereof. Furthermore, the film may contain as an additive 0.1-10.0 wt. % of cellulose powder, which enhances its smokability. Smoking is carried out in the presence of water or water vapor. However, the concrete films described in the examples do not have sufficient water vapor permeability so that the effective loss of moisture is ensured and the sausage product is provided with the necessary consistency.